One way for industrially synthesizing hydrochloric acid uses the direct reaction between the hydrogen and the chlorine in gaseous phase:H2(g)+Cl2(g)→2HCl (g).
This reaction is highly exothermic (about 92 kJ/mol HCl produced) and generates a flame temperature of about 2500° C. to 3000° C. Consequently, the reactor must be cooled constantly, typically by a fluid such as water or steam. The consumption in cooling water of such a unit for directly synthesizing hydrochloric acid is substantial.
Generally, such a method comprises two steps:
(i) the formation of gaseous HCl in a reactor (also referred to as “furnace”) by the reaction indicated hereinabove,
(ii) the absorption of this gas in water in an absorber in order to produce liquid hydrochloric acid.
The furnace for synthesizing and the absorber must be cooled. The same also applies for the liquid acid resulting from the method. The use of the residual heat of the method is an important factor for its economic assessment.
European Patent application EP 1 671 926 A1 (SGL Carbon AG) describes a method for recovering the heat coming from a furnace for directly synthesizing HCl wherein the cooling water is used as a solvent or as a reactant in another process. However, the high-pressure steam is of a more universal use in an industrial site, and it would be desirable to be able to use a maximum of the residual heat of the method for directly synthesizing HCl in this form.
Such reactors are described in many documents (for example, patent application DE 1 08 493 (Siemens Planiawerke)), and such synthesizing units exist on the market.
Patent application FR 2 525 202 (Le Carbone Lorraine) describes a device that aims, via a judicious choice of the temperatures of the cooling fluids in the various zones of the furnace and of the absorber, to improve the heat recovery rate in the form of pressurized steam.
European Patent EP 0 103 863 B1 (Le Carbone Lorraine) describes a unit for synthesizing wherein the burner is located in the top portion, with the flame being directed downwards, and wherein the absorption water of the gaseous HCl trickles on the inner walls of the reactor, becoming enriched progressively with HCl, and wherein the cooling water flows in counter-current and cools the walls of the reactor, which are provided with heat exchange blocks and tubes. Introducing water into the chamber of the reactor generates corrosion problems which call for the use of graphite as construction material for the reactor.
French Patent application FR 2 628 092 (Sigri GmbH) describes a furnace for directly synthesizing hydrochloric acid wherein a heat-transport fluid flows between a heat exchanger which forms the wall of the median segment of the furnace and a steam generator in order to generate saturated steam at a temperature between 170° C. and 230° C. and a pressure of at least 7 bar.
In accordance with the prior art, the furnace and the absorber are made of graphite, as is described in aforementioned document FR 2 525 202. European Patent EP 0 497 712 B1 describes a furnace entirely made of graphite, marketed under the name USL by the company Le Carbone Lorraine.
Historically, ceramic then metal materials were used (see patents DE 506 634 (Rohm & Haas), FR 938 010 (Societa elettrica ed elettrochimica del Caffaro) and DE 857 343 (Chlorberag)). However, the use of metal results in constraints in choosing temperatures inside the furnace. Indeed, according to the teaching of aforementioned document FR 2 628 092, steel can be used for the hottest portion of the furnace, but as soon as there is a risk of water condensation (i.e. formation of liquid hydrochloric acid), graphite must be used in order to prevent corrosion of the elements of the furnace. Graphite is a material that can be machined easily, but pipes arranged in this material do not resist high pressure; in practice this pressure is limited to about 3.5 bars (see for example FR 2 525 202) which corresponds to a steam temperature of about 134° C. The device described in document FR 2 628 092 extracts the heat solely from a median section of the furnace; this section forms a heat exchanger and is made of metal, which allows to obtain higher pressures of about 7 bars.
Document WO 01/25143 (Norsk Hydro ASA) describes a synthesis reactor with recovery of the heat in the form of high-pressure steam; this reactor comprises a furnace section, a convector section and an absorber, and is made from metal materials. This reactor is characterized by an elbow, with the convector section located laterally offset in relation to the furnace, and the absorber located next to the furnace.
In accordance with the description of the patent application, this is not an embodiment but a project, and consequently, very few constructive details are provided.
The presence of an elbow allows for a particularly compact construction and in particular limits the total height of the facility, which lightens its total weight. However, the presence of an elbow requires flanges, which has disadvantages. A flange requires a suitable seal and requires maintenance. A flange always has a risk of a cold spot, and therefore a risk of condensation, leading to a risk of corrosion through the local formation of liquid hydrochloric acid. A horizontal flange finally has a risk of mechanical stress, in particular in the presence of thermal cycles, that can lead to a rupture.
This invention aims to overcome certain disadvantages of known reactors and facilities. A reactor or a facility is sought making it possible to manufacture concentrated hydrochloric acid (up to a concentration of 38%) and to generate saturated steam with a pressure of at least 7 bars (obtained at 143° C.), preferably at least 10 bars (at 165° C.), more preferably at least 14 bars (at 195° C.) and more preferably at least 16 bars (at 201° C.). This reactor or this facility must have a lifespan and maintenance intervals as long as possible, and must therefore resist uniform corrosion as best as possible. In order to be able to operate in optimum safety conditions, it must not have any risk of localized corrosion, in particular through the formation of cold spots, and it must generally be robust, reliable and durable. As such, it is desired to minimize in particular mechanical tension and stress which are reinforced by any thermal cycles, as well as creeping which is able to limit the lifespan of the reactor or of some of its components.